The short answer

Water near a chimney is a path-tracing problem. The visible ceiling mark can be offset from the entry point, and a chimney is a junction between masonry and the roof system. Record the stain after rain, inspect the attic from a safe position, and ask for photos and a separated scope rather than accepting a generic “seal the chimney” treatment.

Separate the possible paths

Area Clue What it does not prove
Step flashing Wetness along the roof-to-masonry edge That the counterflashing is correct.
Counterflashing Open joint or masonry movement That the roof shingles are sound.
Crown or cap Cracks, staining, or water entry at top That water travels straight down.
Shingles and valleys Missing tabs or lifted edges nearby That the chimney is the only source.
Attic Wet sheathing or insulation pattern Whether the first entry is visible.

Photograph from the ground, note wind direction and rainfall, and keep a log of when the stain grows. Do not seal interior drywall or paint until the roof assembly dries. If water reaches electrical equipment or the ceiling sags, protect the room and arrange prompt help.

Ask for a durable estimate

Separate roof repair, flashing replacement, counterflashing, masonry repointing, crown work, attic drying, insulation replacement, and interior finish. A low surface-seal price may exclude the joint or flashing that is actually moving water.

Limits

This guide does not diagnose a roof or chimney from a photo. Roof and masonry work at height belongs with qualified professionals.